Add iban and docs

[martijn00]

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Close #???

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To Do

• Read contributing guide
• Check the original issue to confirm it is fully satisfied
• Add solution description to help guide reviewers
• Add unit test to verify new or fixed behaviour
• If apply, add documentation to code properties and package readme

[deandreamatias]

I remember about a uppercase and lowercase validation to take in account on uppercaseCharLength and lowercaseCharLength regex. Maybe on _alphabetical regex too.

https://github.com/mrmilu/flutter_mrmilu/blob/03fa62cb531b52237b9d035c48654b4ac286136e/lib/src/validators/password_validators.dart#L4

[martijn00]

@deandreamatias Nice! Can you make the new release? Can't wait to try it out!